During the last half century the scientific community has catalogued hundreds of signaling small molecules that are potently regulated via sulfonation ? pheromones, drugs, toxins, steroid and peptide hormones, nuclear- and dopamine-receptor ligands? . Controlling sulfonation of specific metabolites in vivo would allow experimentalists to probe and control sulfur biology with unprecedented precision. Our recent structure/function studies resulted in a robust strategy for preventing sulfonation of a single compound in vivo without altering its receptor interactions or inhibiting sulfotransferases (SULTs). Numerous diseases have been causally linked to sulfonation of individual metabolites. Now, for the first time, we can hope to control these reactions. We recently applied the strategy to an estrogen-receptor agonist and increased its in vivo efficacy ~10,000-fold. We will create, and test in vivo, sulfonation-resistant derivatives of three compounds (two FDA-approved drugs and one endogenous metabolite) with the goal of improving our ability to prevent certain Parkinson's symptoms, control contraception, and regulate thyroid-hormone metabolism. The subject of SULT small-molecule allostery is in its infancy. Eleven of the thirteen human SULT isoforms, each of which operates in separate metabolic domain, harbor one or more allosteric sites. The metabolite- allosteres that bind these sites, and hence pathways linked to the isoforms, remain unknown. We are isolating biomedically relevant, isozyme specific SULT allosteres from small-molecule human-tissue and microbiota libraries, bioactive screening libraries, and in-silico metabolite libraries. We have recently discovered that tetrahydrobiopterin (THB), an essential cofactor in catecholamine neurotransmitter biosynthesis, is a potent, highly specific allosteric inhibitor of SULT1A3, which inactivates neurotransmitters. The sulfonation-dependent regulation of neurotransmitter activity in human brain recommends the THB pocket as a novel neuropharmacological target. We have developed methods that allow SULT ligand-binding site structures to be determined in a matter of days from a ligand's 1D NMR spectrum. We are using these structures, in conjunction with MD-modelling and protein-function studies, to understand the molecular basis of allostere function with the goal of creating compounds that can inhibit, activate and change the substrate specificities of individual SULT isoforms. Using these methods, we have created the first ?man-made? SULT allosteric inhibitor ? a potent, highly selective SULT1A3 inhibitor. The constant undercurrent of protein-function studies in this laboratory is a well-spring of discovery. We are now revealing that promiscuous, half-site enzymes (e.g., SULTs) conformationally couple the energetics of their disparate reactions to one another ? a finding whose implications transcend sulfuryl-transfer metabolism. In addition, we are creating cutting-edge models of SULT allostery and catalysis, and we intend to apply our expertise to the UDP-glucosyltransferases ? the other major phase II enzyme system.